The invention concerns a filter apparatus having an activated carbon molded body with a honeycomb structure, a connection element at the feed flow end and a connection element at the discharge flow end, and a sealing element which sealingly connects the two connection elements and in which the activated carbon molded body is enclosed, wherein the sealing element is formed by a shrink tube into which the activated carbon molded body is shrunk and which is shrunk sealingly on to the connection elements.
In accordance with the new LEV II standard (Low Emission Vehicle) for the emissions from motor vehicles, which applies as from the year 2004 in California and in what are referred to as the Green States of the USA for new vehicles, the level of emission of pollutants may only amount to a maximum of 30% of the emission levels which are allowed there today. In addition in California as from the year 2003 ten percent of the vehicle fleet of an automobile manufacturer must satisfy the ZEV standard (Zero Emission Vehicle). That statutory regulation is giving rise to problems for the manufacturers of tank venting systems. Known tank venting systems are equipped for example with an activated carbon loose bed filter through which the air displaced from the tank flows during the procedure of filling the vehicle tank. In that situation the activated carbon loose bed filter absorbs the fuel in the displaced air. During vehicle operation the flow through the activated carbon loose bed filter is then in the opposite direction and the filter experiences desorption again. The problem now is that after a tank-filling operation the vehicle is switched off at any later time so that the activated carbon loose bed filter cannot be desorbed. In that case the fuel diffuses in the activated carbon loose bed from the charged region into the less heavily charged or uncharged region on the discharge flow side, that is to say to the outlet of the tank venting system. Having arrived there the fuel desorbs out of the activated carbon and is discharged into the environment through the outlet of the venting system.
A commercially available average activated carbon loose bed filter with about 2.7 cm3 activated carbon, after a tank has been completely filled, discharges about 150 mg into the environment in 24 hours. That emission has to be reduced to below 50 mg in accordance with the LEV standard, to below 20 mg in accordance with the ULEV standard (Ultra Low Emission Vehicle) and to 0 mg in accordance with the ZEV standard.
To resolve the above-described problem, additional small activated carbon filters have been developed, which in the stationary, that is to say parked condition of the automobile, absorb those emissions and which are desorbed with clean air during vehicle operation-like the activated carbon loose bed filters discussed above. That additional small activated carbon filter has hitherto been designed in three configurations, more specifically in the form of an activated carbon loose bed filter comprising fine activated carbon granular material, in the form of non-woven fabric which is coated with activated carbon and which is wound up to form a cylinder, or in the form of an activated carbon molded body with a honeycomb structure. Out of those three design configurations, such an activated carbon molded body with a honeycomb structure has the markedly lowest pressure drop. For that reason there is no need for that activated carbon molded body with a honeycomb structure to be shut down during the tank-filling process by means of a valve. That represents a substantial advantage in comparison with an additional small activated carbon loose bed filter comprising fine activated carbon granular material. A further advantage of such an additional small activated carbon filter of the last-mentioned kind with a honeycomb structure is that a correspondingly fine-cell structure provides a markedly greater macroscopic surface area and a higher proportion of activated carbon per unit of volume, than in the case of a non-woven fabric which is coated with activated carbon. An activated carbon molded body with a honeycomb structure therefore represents a so-to-speak ideal solution for the problem described hereinbefore.
U.S. Pat. No. 5,914,294 describes a process for the production of an activated carbon molded body with a honeycomb structure and an activated carbon molded body produced in accordance with that process. Such an activated carbon molded body with a honeycomb structure is also described in the applicants"" prior patent application No. 101 04 882.3. Above-mentioned U.S. Pat. No. 5,914,294 states that, for uses in the automobile sector, to achieve the necessary mechanical stability, the proportion of activated carbon is between about 25 and 35% by weight (column 9, lines 4 through 8). In order to produce a filter with such a relatively small proportion of activated carbon, which is suitable for resolving the above-described problem, the filter must be of a certain minimum size. In order to reduce the volume or the weight of the filter, an increase in the proportion of activated carbon would be necessary to achieve the required adsorption properties. That however results in problems in regard to the mechanical load-bearing capability of that known filter. In order to withstand the extreme mechanical loadings in a vehicle the activated carbon molded body with honeycomb structure therefore either has to be of a very stable nature with a certain minimum size, or it has to be mounted in a suitably vibration-damped manner. Furthermore the entire filter has to be optimally sealed off in relation to the environment in order to reliably prevent diffusion effects through very low levels of leakage. Irrespective of their design configuration, all of the above-described known filter systems suffer from problems which cannot be simply disregarded, in terms of complete and homogeneous sealing integrity thereof. In accordance with the ZEV standard no leakage may occur during the entire lifetime, that is to say the period of use of the filter. In that regard the problems relating to sealing integrity involve not only sealing off the filter body in relation to the environment, but in particular also sealing off the connections.
A filter apparatus of the kind set forth in the opening part of this specification with an activated carbon molded body with a honeycomb structure, a connection element at the feed flow end and a connection element at the discharge flow end and a sealing element which sealingly connects the two connection elements and in which the activated carbon molded body is enclosed, wherein the sealing element is formed by a shrink tube in which the activated carbon molded body is shrunk and which is shrunk sealingly on to the connection elements is known from DE 38 38 426 C2. There the activated carbon molded body is formed by monoliths of metal or ceramic. The monoliths are connected to shielding funnel-shaped portions for the exhaust gas feed and discharge connections of an external housing and to each other by means of shielding rings, thus affording a stiff structure. The shrink tube there does not form a sealing element but an assembly aid.
The object of the present invention is to provide a filter apparatus of the kind set forth in the opening part of this specification with improved filter properties, wherein full-area, homogeneous and optimum sealing of the activated carbon molded body with a honeycomb structure is reliably effected in a simple manner relative to the exterior, that is to say towards the environment, and in particular also towards the connections, while at the same time a good vibration damping effect is achieved.
In accordance with the invention in a filter apparatus of the kind set forth in the opening part of this specification that object is attained in that the activated carbon molded body is definedly spaced from the connection elements so that constricted transitions in the shrunk shrink tube are provided between the activated carbon molded body and the connection elements.
A shrink tube of that kind is inexpensively available, and it can also be processed easily, that is to say using simple means. The filter apparatus according to the invention, with the sealing element which is formed by a shrink tube, enjoys the advantage of ensuring full-area and homogeneous sealing of the activated carbon molded body having a honeycomb structure, in relation to the exterior, and in particular in relation to the connections of the filter apparatus. The constricted transitions of the shrunk shrink tube between the activated carbon molded body and the connection elements provides that the activated carbon molded body is reliably fixed in vibration-damped manner between the connection elements.
The desired characteristics of the vibration damping can be adjusted by the modulus of elasticity of the shrunk shrink tube and/or by the dimensioning of the spacing between the connection elements, that is to say the free constricted connecting portions of the shrunk shrink tube between the connection elements and the activated carbon molded body.
In accordance with the invention flexible adaptation of the filter apparatus according to the invention to installation factors and aspects of a vehicle is possible by the activated carbon molded body having a honeycomb structure comprising at least two activated carbon body portions which are shrunk in mutually spaced relationship in the shrink tube, wherein constricted transitions are formed in the shrunk shrink tube between adjacent activated carbon body portions.
To provide such a filter apparatus according to the invention of the last-mentioned kind, the activated carbon body portions and the two connection elements are introduced into a shrink tube and the shrink tube is shrunk by subjecting it to the action of hot air, in which case the shrink tube bears snugly and sealingly against the activated carbon body portions and the connection elements. A stretching operation is then effected in the still hot condition, that is to say the connection elements are moved away from each other, in which case the activated carbon body portions also move away from each other. In that case constricted concave transitions in the configuration of paraboloids of revolution are produced between the activated carbon body portions. In that case the spacing between the activated carbon body portions can be adjusted by the degree of stretching and by virtue of the overall length of the shrink tube used. The stretched filter apparatus is then quenched with cold air or with cold water so that the shrink tube retains its shape. The flexibility of the filter apparatus according to the invention produced in that way can be adjusted by the modulus of elasticity (E-modulus) of the shrunk shrink tube.
The filter apparatus according to the invention can be used to particular advantage in a tank venting system of a vehicle.